The general trend in electronic equipment and products is towards faster and smaller products. As a result, heat dissipation has become an increasing concern as hotter components are placed into smaller packages and form factors. Excessive heat is linked to numerous types of component failures that result in reduced longevity, and partial or catastrophic failure.
There are a number of design parameters that are used to dissipate heat and otherwise cool the electronic components. Heat sinks and heat pipes are used as passive mechanisms to remove heat or at least quietly disseminate the heat. However in many electronic devices, active cooling is required. Fans have proven to be an excellent tool to generate airflow and maintain a satisfactory environment in many electronic products. Larger products or those with greater heat generation sometimes use multiple fans to establish an optimum environment.
There are a number of problems associated with fans, including acoustic noise, power consumption, and reliability. Designers are always trying to optimize fans by balancing various factors for a particular product to establish the lowest power and the least noise while still maintaining a cool environment. It is well established that proper fan speed control not only extends the life of the fan but is more efficient in terms of power requirements.
A common type of fan in electronic products is the brushless DC motor fan. Such fans operate in much the same manner as DC electric motors, and employ a stator/rotor configuration. The brushless DC motor relies on an external power drive to perform the commutation of stationary winding on the stator, wherein the changing stator field causes the permanent magnet rotor to rotate. Electronically commutated brushless DC motor systems are typically used as drives for blowers and fans used in electronics, telecommunications and industrial equipment applications. There is a wide variety of different brushless motors for various applications.
Fans are typically categorized as 2-wire fans, 3-wire fans, and 4-wire fans. The 2-wire fan has power and ground terminals, and is generally controlled by adjusting either the dc voltage or the pulse width. As there is no tachometric signal, there is no simple indication of the fan speed. A 3-wire fan has power, ground, and a tachometric output. A 3-wire fan can be controlled using the same kind of drive as a 2-wire fan such as a variable dc or a low-frequency PWM, and uses the tachometric signal to check fan speed. A 4-wire fan has power, ground, a tachometric output, that can be employed with a pulse width modulated (PWM) drive input. The PWM uses the relative width of on/off pulses in a train to adjust the level of power applied to the fan motor.
In addition to having various types of fans, there are an associated variety of control systems for the fans. While the fan can be operated with no control aspects, it is inefficient and generates considerable noise by simply operating at full speed. Typically, there are two speed control methods, supply Voltage control and PWM at frequencies above 20 KHz. The Low frequency PWM is not generally used in the industry.
The supply voltage controls the DC voltage applied to the fan motor thereby controlling the fan speed. For quieter operation and lower air flow, the voltage is decreased. If more cooling is required, the voltage is increased which increases the cooling capability and also the noise. As is well known in the art, there are limitations to the relationship of fan speed and input voltage such as startup voltage and stalling problems as well as power inefficiencies.
The majority of small personal computer fans employ PWM controls wherein the voltage applied to the fan is either zero or full voltage switched at >20 KHz to eliminate audible noise.
The cooling fan audible noise is an objectionable by-product of moving air to produce convection cooling. There are several major sources of audible noise in fans, including blade noise, “second harmonic cogging”, as well as commutation noise. The blade noise and cogging cannot be appreciably influenced by electrical designs. However, it is possible to reduce commutation noise via improved circuit design. A circuit design that slows down the current transitions produces less noise.
As noted, the speed of the fan is one of the factors related to the fan noise. At low revolutions per minute (RPM), for example 1400 RPM, the fan audio noise and RFI noise are typically fairly small. At such lower RPMs, the noise is almost entirely comprised of commutation noise. At higher fan speeds, the blade noise begins to contribute to the total fan noise which typically increases significantly at higher RPMs. The blade noise is generally controlled by mechanical design. It is understood that other fans have different requirements and conditions as well as different blade noise starting points.
There have been many attempts at reducing noise generated by cooling fans. These methods may include both mechanical and electrical techniques that seek to provide proper cooling but at a lower power and noise. For example, the linear drive has been enhanced using operational amplifiers or capacitors for bypassing the motor coil as one mechanism to reduce fan noise. Other implementations describe noise reduction by blade design and other mechanical layouts. High frequency PWM schemes keep the fan noise above the human audible range.
What is needed is an improvement to the fans used for heat removal. Such a system should allow for the benefits of the linear drive control in terms of low noise but without the more excessive power consumption.